Testing Abstract Classes

That's a good question. I've been looking for this too.
Luckily, PHPUnit already has getMockForAbstractClass() method for this case, e.g.

protected function setUp()
{
    $stub = $this->getMockForAbstractClass('Some_Abstract_Class');
    $this->_object = $stub;
}

Important:

Note that this requires PHPUnit > 3.5.4. There was a bug in previous versions.

To upgrade to the newest version:

sudo pear channel-update pear.phpunit.de
sudo pear upgrade phpunit/PHPUnit

Unit testing of abstract classes doesn't necessary mean testing the interface, as abstract classes can have concrete methods, and this concrete methods can be tested.

It is not so uncommon, when writing some library code, to have certain base class that you expect to extend in your application layer. And if you want to make sure that library code is tested, you need means to UT the concrete methods of abstract classes.

Personally, I use PHPUnit, and it has so called stubs and mock objects to help you testing this kind of things.

Straight from PHPUnit manual:

abstract class AbstractClass
{
    public function concreteMethod()
    {
        return $this->abstractMethod();
    }

    public abstract function abstractMethod();
}

class AbstractClassTest extends PHPUnit_Framework_TestCase
{
    public function testConcreteMethod()
    {
        $stub = $this->getMockForAbstractClass('AbstractClass');
        $stub->expects($this->any())
             ->method('abstractMethod')
             ->will($this->returnValue(TRUE));

        $this->assertTrue($stub->concreteMethod());
    }
}

Mock object give you several things:

• you are not required to have concrete implementation of abstract class, and can get away with stub instead
• you may call concrete methods and assert that they perform correctly
• if concrete method relies to unimplemented (abstract) method, you may stub the return value with will() PHPUnit method

It should be noted that as of PHP 7 support for anonymous classes has been added. This gives you an additional avenue for setting up a test for an abstract class, one that doesn't depend on PHPUnit-specific functionality.

class AbstractClassTest extends \PHPUnit_Framework_TestCase
{
    /**
     * @var AbstractClass
     */
    private $testedClass;

    public function setUp()
    {
        $this->testedClass = new class extends AbstractClass {

            protected function abstractMethod()
            {
                // Put a barebones implementation here
            }
        };
    }

    // Put your tests here
}